Watts to kWh calculator
Turn a power rating in watts and a run time into kilowatt-hours — the unit on your electricity bill — or work out how long a device can run on a given number of kWh.
Result
Energy used
Result
Run time
How to use this tool
- Energy used: enter the power in watts and how long it runs, in hours (30 minutes = 0.5).
- Run time: switch direction and enter the kWh available — from a battery, power station or daily budget — and the device's watts.
- Multiply the kWh by your unit rate for a cost, or use the kWh cost calculator.
Assumptions
- Assumes constant power draw for the whole period.
- For a battery or power station, usable energy is usually less than the rated capacity because of conversion losses; enter the usable figure if you know it.
Technical details: what the fields mean
- Power
powerWatts - Power in watts, from the appliance label or a plug-in energy monitor. 1 kW = 1000 W.
- Duration
hours - How long it runs, in hours. Minutes ÷ 60 gives hours; a year is 8,760 hours.
Methodology
Energy (kWh) = power (W) ÷ 1000 × hours. Run time (hours) = kWh × 1000 ÷ power (W).
Worked example
1000 W appliance running for 30 minutes
Inputs: Power: 1000 W, Duration: 0.5 hours
Result: 0.500 kWh (1000 ÷ 1000 × 0.5).
2000 W for 3 hours
Inputs: Power: 2000 W, Duration: 3 hours
Result: 6.000 kWh
A 5 W device left on standby all year
Inputs: Power: 5 W, Duration: 8760 hours
Result: 43.800 kWh a year — small per device, but it adds up across a house.
Run time: a 1 kWh power station running a 60 W fridge
Inputs: Energy available: 1 kWh, Power: 60 W
Result: 16.67 hours at a constant 60 W. A fridge cycles on and off, so it would usually last longer.
Run time: 1.5 kWh for a 2 kW heater
Inputs: Energy available: 1.5 kWh, Power: 2000 W
Result: 0.75 hours — 45 minutes.
Practical guidance
- Use this alongside the kWh cost calculator to turn a device's wattage and run time into a cost.
- A plug-in energy monitor measures kWh directly over a day or week, which is more accurate than the label wattage for appliances that cycle.
Common mistakes
- Forgetting to convert watts to kilowatts (÷1000) before multiplying by hours — this calculator does that step for you.
- Entering minutes as hours: 30 minutes is 0.5, not 30.
FAQs
How do I convert watts to kWh?
Divide the wattage by 1000 to get kW, then multiply by the number of hours it runs.
How long will a 1 kWh battery run a 100 W device?
About 10 hours at a constant 100 W, less any conversion losses in the battery or inverter.
Is kW the same as kWh?
No. kW is the rate of using power at a moment; kWh is the energy used over time. A 2 kW heater uses 2 kWh in one hour.
Last reviewed: 2026-10-11
Assumes constant power draw, which isn't true for appliances that cycle or vary their power.
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